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CD4046B: CD4046 VCO frequency precision

Nicolas Cormier
Prodigy 40 points
Part Number: CD4046B
Other Parts Discussed in Thread: CD4541B

Tool/software:

Hello community,
I would like to use the CD4046BPWR for a new application in variable frequency generation from 100 to 3000Hz.
I can't find in the datasheet the intrinsic precision of the VCO in this frequency range.
What is it?
Only the maximum frequency condition is given for 10kohms and 50pF with a ratio of 2 between min/typ frequency values. I don't think (I hope) that this tolerance applies over the whole range.

Is there a mathematical law that determines R and C rather than charts?

Thank you for your help.

Sincerely

Nicolas

  • 0
    TI__Expert 8810 points

    Hi Nicolas, 

    Generally speaking, the 4046 PLL family is operated in a closed loop so the VCO frequency is set with feedback from another source. The VCO frequency may not be well defined aside from the typical charts as you noted. 

    Would something like a 555 timer work for your application? I believe those should have a more deterministic relationship between the output frequency and value of the passive components. 

    Regards, 

    Connor 

  • 0 in reply to Connor Lewis
    Prodigy 40 points

    Hi Connor,

    Thanks for this prompt reply.

    If I understand correctly, the CD4046 alone is not recommended for +/-3% oscillator accurancy. ( 100 to 3000Hz band ).

    But do you have any idea of the accuracy of this VCO in all conditions?

    Sincerely

    Nicolas

  • 0 in reply to Nicolas Cormier
    TI__Expert 8810 points

    Hi Nicolas, 

    Correct, I would not recommend CD4046B for an application that requires +/- 3% frequency accuracy unless you are able to lock it to another source. I'm not sure what the worst case accuracy of the VCO is in the 100Hz-3kHz range, but like you pointed out the maximum operating frequency changes by a factor of 2 between the minimum and typical spec (I believe it's mostly affected by operating temperature). 

    Besides a 555 timer, CD4541B could be another option. The output frequency tolerance is mainly determined by the accuracy of the passive components you use, so +/- 3% accuracy should be achievable. 

    Regards, 

    Connor